Energy absorber

ABSTRACT

An energy absorber comprises a helically coiled stores of at least partially yielding material that is drawn over a roller, or two rollers on perpendicular rotational axes, and securely fixed to a base that is attached to fragile structure such as a roof so that when an external force of sufficient size is applied to the roller or rollers through a connecting attachment eye the coil together with roller or rollers moves away from the attachment to the base whilst the coiled material is deployed over the roller or rollers. The absorber may be housed in a casing that is fixed to the base such that the strength of the fixing to the base is sufficient to withstand a pre-determined load after which the fixing fails and the housing moves with the coiled store providing protection for both the coiled store and fragile structure.

This invention relates to an energy absorber and in particular to anenergy absorber intended for attachment to relatively fragile structuresfor use in applications such as height safety equipment.

BACKGROUND OF INVENTION

Height safety equipment is intended to prevent fall injuries topersonnel working at height. A common arrangement for height safetyequipment is for a safety rope or cable to be attached between endanchor points in an area to which access is required. Intermediateanchor points can then be added along the length of the cable to reducefall distances before arrest and also to enable a change in directionalong the length of a cable between end anchor points such that thecable can be routed around corners or some other direction. Personnelwear harnesses connected to travellers that are attached to and able tomove freely along the safety ropes or cables. More sophisticatedtravellers are able to transverse intermediate anchor points withoutbecoming detached from the cable.

The loads likely to be applied to anchor points depend on variousfactors. Probably the most significant of these is whether the heightsafety equipment is a restraint system intended simply to restrainpersonnel against the possibility of falling or whether the heightsafety equipment is a fall arrest system intended to arrest personnelsafety immediately following a fall. Fall arrest systems allow personnelto access areas close to edges of building or tall structures whereasrestraint systems limit movement of personnel to safe access areas wherethere is no possibility of a vertical fall. Therefore, maximum likelyloading on anchor points in restraint systems is substantially less thanthat in fall arrest systems. This invention is intended for use in fallarrest systems although it could also be used in restraint systems.

One example of a commonly accessed fragile structure is roofs onrelatively tall buildings. Fall arrest systems are installed to enableaccess to areas where falls are possible such as gutters and areas closeto roof lights. Many such roofs exist on commercial buildings where theroof structure comprises pieces of relatively thin steel sheet formedwith regular ridges that run typically from the apex of the roof alongits pitch to the edge. Each piece is usually attached to adjacent piecesand all pieces are fixed to a system of lightweight formed steel beamsthat provide rigidity over the area of the roof and attach the roof tothe surrounding ground bearing structure such as walls, pillars orexternal suspending structures. Whilst such roofs are intended towithstand the worst anticipated weather conditions they are not designedto cope with the relatively high localised loading that may betransmitted through fall arrest system anchor points.

The attachment of anchor point brackets to roofs by fall arrest systeminstallers is most conveniently achieved by screwing the bracketsdirectly to the roof sheets. This avoids the need to access the internalroof structure from outside the roof and also utilises the attachmenttechniques most commonly applied in the roof installation industry.Further, this allows the anchor point locations to be determined solelyby the requirements of the fall arrest system because the anchor pointscan be located anywhere on the roof surface and are not limited tolocations where they can be attached to the beams.

The structural strength of roofs formed of roof sheets is not verygreat. Therefore, where fall arrest system anchor points are attached tothe roof sheets directly rather than to the support structure, it isimportant that the loading applied to the roof structure is limited.

Further, whilst screws used in roofing tend to provide good grip inshear, they pull out relatively easily in tension largely as a result oftearing in the thin steel roof sheet. Therefore, it is important wherefall arrest system anchor points are attached to a roof, particularlywith screws, that loading on the anchor point attachment to a roof islimited and primarily applies shear forces to attachment screwsirrespective of fall conditions such as fall distances and number ofpersonnel falling.

In order to arrest personnel safety following a fall, a fall arrestsystem needs to absorb all fall energy safely and without subjectingpersonnel to arresting loads greater than maximum safe values, which arespecified by industry and international regulations. Also, mostinternational regulations require that any load on any part of thesystem following the most demanding fall conditions for which a systemis designed should never be greater than 50% of the load at which such apart fails. This safety factor is also applied to anchor pointattachments and their supporting structures such as a wall or roof.

Cable such as steel wire commonly used in cable based fall arrestsystems has very little elastic stretch and therefore absorbs littlefall energy in the event of arresting a fall unless end anchor pointloading is able to be relatively high. Personnel wear energy absorbinglanyards to limit arresting loads on personnel and these will assist tosome extent in absorbing fall energy, although the deployment force ofthe lanyard absorbers is relatively low and also the deployment extenthas the effect of adding to the fall distance and therefore the fallenergy. However, anchor point loading on relatively fragile structuresneeds to be limited to a maximum of 50% of the strength capability ofthe structure itself. End anchor point loading in the event of fallenergy absorption by elastic stretch alone would easily exceed 50% ofthe capability to failure to anchor point attachments to many roofs.Anchor point loading at a change in direction of the cable becomessignificantly greater even than end anchor point loading. For example,an anchor supporting a ninety degree change in the direction of thecable would need to support a load increased by a factor to the squareroot of two.

A further condition that would cause high loading at an anchor point iswhere a multiple personnel fall occurs close to or on an intermediateanchor point. Initially, the multiple fall energy would need to beabsorbed largely by reluctant extension at the anchor point itself toavoid high anchor loading. This depends on the degree of reluctance andextension. If the extension is low then loading at the anchor will becorrespondingly high.

One problem encountered with energy absorbers, particularly for thoseused in fall arrest systems, is that the position at which a fall willoccur relative to anchor point brackets cannot be predicted so thatenergy absorbers must be able to operate effectively for a fall arrestload being applied from a range of directions. Further, in order toallow economies of scale to be achieved by the use of common componentsthroughout fall arrest systems and to avoid the possibility that a fallarrest system could be rendered ineffective by mounting an anchorbracket in the wrong orientation it is desirable to provide an energyabsorber able to operate over a wide range of directions of appliedload.

Accordingly, a first object of the invention is to provide an energyabsorber capable of limiting loading to a known and safe value at itsattachment to a structure irrespective of the direction of the loading.

A further object of the invention is to provide an energy absorber ableto absorb a maximum or optimum amount of energy for a given extensionand loading limit.

BRIEF SUMMARY OF THE INVENTION

In a first aspect, this invention provides an energy absorber comprisingmeans for attaching the energy absorber to a supporting structure, meansfor attaching the energy absorber to a load element and orienting meansresponsive to a first predetermined tensile load applied to the meansfor attaching to a load element to change the orientation of the energyabsorber towards the direction of an applied load, and furthercomprising a store of plastically deformable material and deploymentmeans responsive to a second predetermined tensile load greater than thefirst effective to deploy said plastically deformable material in acontrolled manner whereby said material is permanently plasticallydeformed during said deployment, thereby absorbing energy.

In a second aspect, this invention provides an energy absorbercomprising means for attaching the energy absorber to a supportingstructure, means for securing the energy absorber to a load element, asubstantially cylindrical casing, and a store of plastically deformablematerial and deployment means contained within the casing, theplastically deformable material and deployment means being responsive toa second predetermined tensile load to deploy said plasticallydeformable material in a controlled manner whereby said material ispermanently plastically deformed during said deployment, therebyabsorbing energy, and further comprising orientation means includingsaid casing responsive to a first predetermined tensile load lower thanthe second to change the orientation of the energy absorber by rotationabout a lower rim of the casing towards the direction of an appliedload.

Energy absorbers according to the invention are able to absorb appliedload energy from a very wide range of directions and is able to absorban optimum amount of energy for a given allowed extension and maximumloading applied to a supporting structure.

Where the energy absorber is incorporated into an anchor point for afall arrest system the anchor point is able to be used as an end anchor,corner anchor or intermediate anchor. Further, given the maximum anchorpoint load limitation, use of the energy absorber according to theinvention enables an optimum or near optimum fall energy performance interms of intermediate anchor spacing, fall distance and number ofpersonnel in a multiple fall situation.

This is achieved primarily by providing sufficient constant or nearconstant force energy absorption at each anchor point so that anchorpoint loading is limited to the designed constant deployment forceirrespective of system fall conditions. Furthermore, the absorber ineach anchor point is capable of deploying in any direction substantiallyin the plane of the surface to which it is attached in order to providefor the multi-directional possibilities required in the various anchorpoint applications such as end anchor, intermediate anchor and change ofcable direction anchor. This invention also includes the constant forceabsorber incorporated into a bracket for fixing to a fragile structure.

BRIEF SUMMARY OF THE DRAWINGS

The invention will now be described by way of example only withreference to the accompanying diagrammatic figures, in which:

FIG. 1 shows a partially cut away view of an energy absorber accordingto a first embodiment of the invention;

FIG. 2 shows an elevational partially cut away view from a directionperpendicular to FIG. 1;

FIG. 3 shows a further sectional view of the energy absorber of FIG. 1with an external casing in place;

FIG. 4 shows the energy absorber of FIG. 1 in operation;

FIG. 5 shows a detail of an end stop arrangement of the energy absorberof FIG. 1;

FIG. 6 shows a partially cut away elevational view of an energy absorberaccording to a second embodiment of the invention;

FIG. 7 shows a partially cut away top view of the energy absorber ofFIG. 6;

FIG. 8 a shows a top view of a bracket arrangement suitable for use withenergy absorbers;

FIG. 8 b shows an end view of the bracket of FIG. 8 a;

FIG. 8 c shows a detail of the bracket of FIG. 8 a;

FIG. 9 shows an alternative bracket arrangement suitable for use withenergy absorbers;

FIG. 10 shows a partially cut away side elevation view of an energyabsorber according to a third embodiment of the invention;

FIG. 11 shows a partially cut away side elevational view from adirection perpendicular to the view of FIG. 10;

FIGS. 12 a to 12 c show views of a yoke used in the energy absorber ofFIG. 10;

FIG. 13 shows a guide bracket used in energy absorber of FIG. 10;

FIG. 14 a shows a top view of a base plate suitable for use with theenergy absorber of the third embodiment; and

FIG. 14 b shows a cross sectional view through the base plate of FIG. 14a.

Referring to FIGS. 1 to 5, a first embodiment of the invention is shown.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION

In the energy absorber of the first embodiment coiled store 1 is ahelically wound coil store of at least partially yielding material 3such as stainless steel rod, one end of which is passed around roller 4and rigidly fixed to raised feature 10 on base plate 11. Base plate 11is rigidly attached to a suitable location on a high building orstructure as part of a fall arrest or some other system requiring energyabsorption. The base plate 11 may be directly attached or may beattached through a suitable bracket or other element.

Roller 4 is free to rotate about pin 5 with the axis of rotation beingparallel to the axis of the helical coil. Pin 5 is attached to adeployment structure comprising connected plates 6, 7 and 8 and alsoload pulling eye 9. The plates 6 and 7 are parallel and spaced apart oneither side of the roller 4 to hold the roller 4 between them and definea channel through which the material 3 from the coiled store passes. Thepin 5 is attached to the plates 6 and 7. The plate 8 links the plates 6and 7 to the load pulling eye 9. Fall arrest system cable 12 passesthrough the load pulling eye 9. A guide 14 is rigidly attached to plate7 and projects into the inside of the helical coil.

When an increasing load is applied to eye 9 in any direction above baseplate 11, plate 8 tends to straighten together with the part 3 a ofhelical coil material 3 between roller 4 and raised feature 10 until theload is sufficient to begin pulling and yielding coiled material 3 overroller 4. The guide 14 projects from the plate 7 into the interior ofthe coiled store 1. Guide 14 is formed by a roller and is in contactwith the inner surface of the coil on the opposite side of the roller 4to the entry direction of the coil material 3 when pulling of the coilmaterial 3 over the roller 4 occurs. Guide 14 counteracts the tendencyfor the axis of the helical store to move relative to the axis of roller4 and therefore ensures a constant degree of yielding of material 1.

Typically, for use in a fall arrest system the load required to beginyielding and deployment of the material 3 is about 10 kN. This providesthe desired safety margin for a typical roof able to support a maximumload of about 20 kN.

The yielding of the coiled material 3 as it passed over roller 4 isarranged to be plastic deformation, allowing the yielding coiledmaterial to absorb large amounts of energy as it deploys from thehelical store 1.

Guide 14 is shown as operating within the helical coil in order toachieve a compact absorber but a guide could alternatively be locatedoutside the coil.

In practice, the force required to be applied at eye 9 to initiate andcontinue yielding of material 3 in this manner has been found to remainsubstantially constant as the absorber deploys. On deployment, thehelically wound coil 1 moves with roller 4 and the other parts of thedeployment structure away from base 11 as the coiled material 1 unwindsaround roller 4.

FIG. 3 shows the absorber housed in a casing 15 which is attached to thebase 11 by fastenings 16 and 17. Note that the casing 15 is not shown oris shown only in part in FIGS. 1 and 2. In cable based fall arrestsystems on structures such as roofs, the cable needs to be spaced wellclear of the roof to avoid such things as sagging cables and fall arrestsystem travellers and shackles from damaging the roof surface. Casing 15assists in supporting the absorber to provide the necessary spacing andprotects the absorber components from the weather and otherenvironmental effects. However, a problem with this spacing requirementis that when load is applied at eye 9 in a direction substantiallyparallel to the plane of a roof, the torque applied on the roof andfixings between the energy absorber and the roof due to the couplegenerated by the separation between eye 9 and the roof can become toohigh, tending to twist roof sheeting and to apply an undesirable pullout load on screws fixing the absorber to the roof. In order to overcomethis problem the absorber is able to rotate about the raised feature 10by bending of the section 3 a of material 3 until the absorber is inline with the direction of the force applied to the eye 9. The coupleacting on the roof and fixings is then reduced to the couple generatedby the much smaller distance between the roof and the top of the raisedfeature 10, greatly reducing the torque applied on the roof and fixings.This makes the pull out force applied to any screws relatively small,most of the load acting as a shear force.

In order to prevent bending of the material 3 under low loads the casing15 supports the absorber, holding the eye 9 in a fixed position relativeto the base 11. The casing 15 is designed to resist becoming detachedfrom base 11 until the load on eye 9 has reached a pre-determined size.At this point the fastenings 16 and 17 fail and allow casing 15 to partfrom base 11. This predetermined detachment load is designed to besufficiently low to avoid significantly damaging the roof and is lowerthan the load required to deploy the material 3 from helical store 1over the roller 4. Further increases of load on eye 9 may then reach asize where the absorber begins to deploy such that casing 15 follows themovement of helical store 1 and eye 9 away from the base 11 ondeployment, as shown in FIG. 4. Factors determining the choice of thispre-determined load at which casing 15 parts from base 11 are partly todo with avoiding accidental damage to the absorber and bracket bypersonnel, particularly because personnel may use the absorber andbracket as a convenient purchase point for maintaining their balance orarresting a slip (as opposed to a fall). However, there is also theimportant consideration of withstanding the pre-tensioning of cable 12suspended between absorbers and the possibility of personnel applyingaccidental loading on the cable leading to the possibility of absorbersand brackets leaning prematurely.

Essentially the predetermined parting load should be high enough thatparting will only occur in a fall arrest situation and low enough not tocause damage to the roof or fixings. A parting load of about 2500 N(2.5kN), that is, approximately one quarter of the deployment load, has beenfound to be effective in practice.

A further factor to be considered in deciding the spacing of eye 9 abovea fragile structure such as a roof is that upon detachment of casing 15from base 11 the rotational movement of eye 9 about the anchor 2provided by the fixing of the end of material 3 to base 11 translatesinto a corresponding increase in the extent to which a person fallsvertically in the event of an arrested fall. This extent needs to beminimised to reduce the total fall arrest energy which must be absorbedand also the distance fallen before a fall is arrested, particularlywhere the distance between the edge of a roof or fragile structure andthe ground is relatively small. Therefore, it is desirable to arrangethe position of anchor 2 and raised feature 10 above the roof or fragilestructure at a distance where the resulting torque on the roof orfragile structure is sufficiently low to avoid damage and indeedsufficiently low to avoid failure of the roof or fragile structure whentwice the load on eye 9 that initiates yielding and movement of material3 is applied to the roof or fragile structure. This problem is addressedby locating anchor point 2 above the roof or fragile structure so thatthe resulting torque on the roof or fragile structure is close to thestrength limitations of the roof or fragile structure to avoid givingaway excessive effective fall arrest line extension without significantresistance and therefore without energy absorption.

Further, casing 15 provides a protective shell between the absorber androof during absorber deployment because the absorber parts are containedwithin the casing 15 as shown in FIG. 4. Whilst this is important toavoid or minimise damage to roofing, it also reduces the possibility ofthe absorber on deployment becoming caught on awkward surfaces in thepath of deployment.

Thus, when a fall arrest event occurs the operation of the energyabsorber is as follows. The energy absorber remains in place withoutmoving until the loading applied to the eye 9 through the cable 12reaches the detachment load. The fastenings 16 and 17 then fail,releasing the casing 15 from the base 11. This separation allows theabsorber to rotate about the anchor point 2 where the material 3 isattached to the raised feature 10 on base plate 11 so that the absorberis oriented in the same direction as the pulling force applied to theeye 9 by the cable 12.

This rotation of the absorber is allowed for by deformation of thematerial 3 in the region 3 a between the roller 4 and the raised feature10, which allows the distance of the eye 9 from the raised feature 10 toincrease slightly.

During a fall arrest event the load applied to the eye 9 through thecable 12 will then increase beyond the deployment load required toplastically deform the coiled material 3 out of the coil 1 and over theroller 4. Under this load the eye 9 and attached parts of the energyabsorber making up the deployment structure will move away from theraised section 10 in the direction of the load applied by the cable 12as the material 3 is deployed out of the coil and around the roller 4.

The plastic deformation of the material 3 as it deploys out of the coilaround the roller 4 absorbs energy, in the case of fall arrest the fallenergy, and generates a substantially constant deployment force, whichin a fall arrest system acts to slow and eventually stop the fall.

Eventually, all of the fall energy will be absorbed in plasticallydeforming the material 3, the fall arrest forces will drop too low todeploy the material 3 out of the coil 1 around the roller 4 anddeployment of the material 3 will stop.

Usually the energy absorber will be designed so that all of the energyexpected to be released in a fall arrest incident will be absorbedbefore all of the material 3 in the coil 1 is deployed. However, it ispossible that unforseen circumstances may cause the fall arrest energyto be greater than expected so that all of the material 3 in the helicalcoil 1 will be deployed.

FIG. 5 shows the operation of an end stop in the event that theabsorber's helical store becomes fully deployed. This end stop isimportant in height safety equipment in order to comply with industryand international standards. Most such standards require that aftercomplete deployment of an energy absorber the absorber should withstandat least twice its working deployment load without failing. Failure inthis context refers to any failure of connecting components between eye9 and the roof or other fragile structure to which base 11 is attached.If a sustained load greater than the deployment force is applied after afall has been arrested, the material 3 will deploy until all of thematerial 3 has been deployed. Accordingly an end stop is required toprevent separation of the eye 9 from the base 11.

In the arrangement shown, an end stop 13 is provided by nut 13 locatedon a threaded end part 3 b of the material 3. The nut 13 is larger thanthe clearances between the plates 6 and 7 and between an edge of plate 8and the roller 4. Accordingly, when the end of the material 3 is reachedduring deployment the nut 13 is unable to pass between the plates 6 and7 and so does not reach the roller 4. However, for safety it ispreferred that the nut 13 is also unable to pass between the plate 8 androller 4 to provide a secondary back up end stop. As can be seen fromFIG. 5 the nut 13 is arrested by contact with plates 6 and 7 while thematerial 3 is in contact with the roller 4.

In this end stop arrangement the material 3 passes around the roller 4beyond the point at which the end stop 13 is stopped by the plates 6 and7. As a result, although the absorber as a whole must be able to supportdouble the deployment load, the corresponding load which the end stop 13is required to resist is lower than this. This is because some of theload supported by the absorber is supported by the “capstan effect” ofthe material 3 passing around the roller 4 and so is not applied to theend stop 13.

Alternatively, end stop 13 can be any structure presenting an enlargedcross section of material 3 such that the enlarged part is held by arestriction provided between plates 6, 7 and 8, or some otherrestriction means.

FIGS. 6 and 7 show a second embodiment of the invention wherein ahelical coil store 20 of partially yielding material 21 has a coil axisthat is substantially parallel to the direction of pull on eye 9. Thisprovides a compact arrangement that can fit easily within a neatcylindrical post with a smaller outside diameter than casing 15 shown inFIG. 3. End 21 a of the partially yielding material 21 is passed aroundrollers 24 and 25. The rollers 24 and 25 are both mounted for rotationon a deployment structure 27 for rotation about mutually perpendicularaxes. The rotational axis of roller 24 is substantially parallel to thewound axis of helical store 20 and perpendicular to the rotational axisof roller 25 such than end 21 a of the partially yielding materialpasses within and through the helical store coils and through a guidehole 28 shown as part of a structure 27 and is then fixed securely tobase 23 at fixing location 22. Guide hole 28 is intended to constrainthe yielding material to assist its alignment with respect to roller 24.The pulling eye 9 is also fixed to the deployment structure 27. Fallarresting cable 12 typically passes through eye 9 as before.

When an increasing load is applied to eye 9 in any direction, typicallyin a fall arrest system the applied load will be parallel to or abovethe plane of base 23, the absorber will tend to lean in the direction ofthe applied load to eye 9, such leaning being determined between eye 9and the attachment at anchor 22 of the end of the partially yieldingmaterial to base 23. Continued increasing of the applied load to eye 9will result eventually in material 21 being pulled around rollers 24 and25 thereby undergoing plastic deformation due to yielding. Guide 26 isan abutment being either part of or fixed to structure 27 thatcounteracts the tendency for helical store 21 to become misaligned withrespect to roller axes 24 and 25 as material 21 yields around rollers 24and 25. As yielding of material 21 is initiated and continues thereafterthe applied pull load on eye 9 tends to remain constant because of theconsistency and predictability of the plastic yielding process. Energyis therefore absorbed, the amount of which is the product of the load oneye 9 as material 21 is yielding and the length of material 21 thatpasses around rollers 24 and 25, allowing eye 9 to move with respect tobase 23.

The embodiment shown in FIGS. 6 and 7 can also be combined with a casing15 as shown in the embodiment in FIGS. 3 and 4 in order to control theapplied load required to begin rotation or leaning of the absorber inthe direction of the applied load and to provide environmentalprotection. Also, the need for a secure end stop after completedeployment of the coiled energy store as shown in FIG. 5 applies to theembodiment shown in FIGS. 6 and 7 and the end stop could be similar tothat shown in FIG. 5 in that there would be an enlargement on the end ofthe material 21 that would be unable to pass through a restrictionformed in structure 27 or some combination of either or both roller 24and 25 or such combination and structure 27.

FIGS. 8 a, 8 b and 8 c show a bracket arrangement for fixing todifferent roof profiles. Whilst use of this bracket is not required touse the absorber and casing it is nevertheless useful in manyapplications requiring the absorber. Roof profiles vary considerablyalthough nearly all profiles tend to feature longitudinal spaced apartand parallel ribs in roofing sheets, which typically run between theridge and the edge of the roof. Roof sheeting is usually made of thinsteel sheet sometimes reinforced for rigidity with material such asrigid expanded foam. The simplest way for fixing a base such as base 11in FIGS. 1 to 4 to roof sheeting is by means of screws directly throughthe roof sheeting itself. However, it has been found that the mechanicalstrength of such screw fixing to roof sheeting is only relatively highwhen the resultant loading on the screws is substantially acting inshear on the screws. Resultant loading acting in tension on the screwstends to cause the screws to break through the thin roof sheet materialrelatively easily. Therefore, it is desirable to ensure as far aspossible that the resultant loading acts in shear on screw fixings.

Roof profile 33 is typical in principle for common roof sheeting havingridges regularly spaced. In order to provide screw fixings 35 where theresultant loading is largely acting in shear on screw fixings 35,bracket 30 enables the screw fixings 35 to be fastened into the slopingfaces of the ridges. A problem with this is that the angle of slope ofthe ridge sloping faces varies widely across different proprietarymodels of roof sheeting. To overcome this problem, fixing bracket 30 hasa concave form as shown in FIG. 8 c allowing fixture to slope anglesvarying from that shown as 36 and 37.

FIGS. 8 a and 8 b show a base 32 to which an energy absorber could befixed. This base has a series of slotted positions that can be alignedin different arrangements with corresponding holes in fixing bracket 30such that the fixing of base 32 to fixing bracket 30 can accommodate themost common different roof types. The main differences in such rooftypes are usually the spacing apart of the ridges, the width of eachridge and the angle of slope of each ridge. Therefore by providing thepossibility of rotating base 32 by 90 degrees to provide two spacingvariations together a variety of slotted holes base 32 can be fixed tobracket 30 to accommodate most common roof section designs.

The base 32 should be rigid enough to support the load at which thecasing 15 separates without buckling. This is because if the base 32buckles before separation occurs, the buckling base plate 32 will applyvery high inward loads to the securing screws and brackets due toleverage.

FIG. 9 shows a further method for attaching a base to a roof and isparticularly suited to resisting loads applied from a positionsignificantly above the surface of base 40 that result in torque loadson the base. Fixing brackets 44, 45, 46 and 47 are similar componentseach fixed to slopes on roof sheeting ridges. Each fixing bracket has ashaped typically straight side hole through which the ends 41 of crossbars 38 and 39 can pass but with sufficient clearance to enable somerotational adjustment of fixing brackets 44, 45, 46 and 47 toaccommodate different roof sheet ridge slope angles. Cross bars 38 and39 are part of or attached to base 40 and protrude beyond the edges ofthe rectangular perimeter surface of base 40 to provide adjustment foraccommodating roof sheet ridges that are positioned apart at differentspacings. Fixing brackets 44, 45, 46 and 47 can then be screw fixed tothe slope surfaces of roof sheet ridges. End stops 43 can be insertedthrough holes 42 located at either end of cross bars 38 and 39 to limitlateral movement of base 40 in the event of lateral loading.

When a load is applied at position significantly above the surface ofbase 40, the resulting loading on cross bars 38 and 39 and the fourfixing brackets is such that loading on the fixing screws in largely inshear. Also, because the load acting between the fixing brackets andcross bars is largely normal to the interconnecting surfaces andtherefore results in relatively high friction between the cross bars andfixing brackets. This helps to limit sliding movement between the crossbars and fixings brackets. This friction effect can be increased byarranging one or more of the interconnecting surfaces to be toothed orto be sharp edged.

Fixing brackets 44 and 45 could be joined together to become onecomponent. This component could also be used to provide the function offixing brackets 46 and 47.

A third embodiment of the invention is shown in FIGS. 10 to 13. Theenergy absorber according to the third embodiment is arranged similarlyto the absorber of the first embodiment with the coiled material axisperpendicular to the load direction.

As can be seen in FIGS. 10 and 11 the energy absorber of the thirdembodiment incorporates a helically wound coil store of a plasticallydeformable deployable material 50. One end 50 a of the deployablematerial 50 passes around first and second rollers 51 and 52 and througha retainer element 53.

The rollers 51 and 52 are arranged for rotation around parallel axesabout respective pins which are attached to a pair of spaced apartparallel plates 54 a and 54 b formed by the two side pieces of asubstantially u-shaped yoke 54. The rollers 51 and 52 are locatedbetween the two side plates 54 a and 54 b of the yoke 54.

The two rollers 51 and 52 are located inside the profile of the coil, inorder to provide a compact energy absorber.

A load pulling eye 55 is attached to an end piece 54 c of thesubstantially u-shaped yoke 54 so that the pulling eye 55 is rigidlyconnected to the rollers 51 and 52.

A guide bracket 56 is attached to the first side plate 54 a of the yoke54. The guide bracket 56 has a projecting guide portion 56 a extendingsubstantially perpendicularly to the side plate 54 a and having a hole56 b therethrough. The diameter of the hole 56 b is sufficient to allowthe yielding material 50 to pass through it and control the movement ofthe yielding material 50.

The yoke 54 together with eye 55, guide bracket 56 and rollers 51 and 52form a deployment structure.

When a sufficient load is applied to the loading eye 55 the yieldingmaterial 50 will be deployed from the helical store by passing aroundthe first roller 51. This bends the material so that it is plasticallydeformed in a first direction. The yielding material 50 then passesbetween the first and second rollers 51 and 52 before passing around thesecond roller 52 so that the material 50 is again plastically deformedbut in the opposite direction. Before reaching the first roller 51 thematerial 50 passes through the hole 56 b in the guide bracket 56 andthis allows the guide bracket 56 to counter any tendency for the axis ofthe helical coil store to move relative to the roller 51 as the material50 is deployed. The yoke 54 and guide bracket 56 are shown in moredetail in FIGS. 12 a to 12 c and FIG. 13 respectively.

Typically, for use in a fall arrest system the load required to beginyielding and deployment of the material 50 is about 10 kN.

The energy absorber is retained on a base plate 57 by the end 50 a ofthe deployable material 50 passing through a hole 57 a in the centre ofthe base plate 57 and then through a retaining element 53. The end 50 aof the deployable material 50 is screw threaded and a nut 58 is screwedonto the end 50 a on the opposite side of the base plate 57 andretaining element 53 to the rest of the absorber to attach thedeployable material 50 to the base plate 57.

Only a single nut 58 is shown, but further retaining elements, such as alocknut, could be added if desired.

Similarly to the first embodiment a casing is provided. In the thirdembodiment the casing is provided by a first substantially hemisphericalcap portion 59 attached to the yoke 54 and pulling eye 55 and asubstantially cylindrical body section 60 extending between the cap 59and the base plate 57. A spacer ring 61 is held between the cap section59 and the body section 60 of the casing. The body section 60 andretainer ring 61 are held in compression between the cap 59 and the baseplate 57, the necessary compression forces being supplied by tighteningthe nut 58 on the deployable material 50 to pretension the energyabsorber.

Typically, a pretension load of about 800N is applied.

As in the first embodiment, the casing supports the energy absorberagainst rotation until a predetermined, rotation, load is reached andprotects the rest of the absorber from environmental effects.

A fall arrest system safety cable will normally pass through the loadeye 55.

When a load is applied to the absorber through the load eye 55 in a fallarrest situation this load will generally be substantially parallel tothe base plate 57. Where the absorber is mounted on a roof, the loadwill generally by substantially parallel to the roof surface to whichthe base plate 57 is attached.

In a fall arrest situation, when a fall arrest event occurs the energyabsorber will initially support the applied load without movement untilthe applied load reaches a first, rotation, level. When the appliedforce reaches the rotation level the body portion 60 of the casing willrotate or pivot about its lower rim where it contacts the base plate 57,allowing the energy absorber to rotate about the lower rim of the bodyportion 60 until the energy absorber is arranged substantially parallelto the load applied to the load eye 55. Then, if the applied loadincreases to a higher second, deployment, level the deployable material50 will begin deploying out of the helical store by moving around therollers 51 and 52, allowing the deployment structure including the loadeye 55 and other attached parts of the energy absorber to move away fromthe connection element 53.

Typically, for use in a fall arrest system, the load requested to beginrotation of the energy absorber is about 2.5 kN.

As the deployable material is deployed out of the helical coil aroundthe rollers 51 and 52 the deployable material 50 is plastically deformedtwice in opposite directions generating a substantially constantdeployment force and absorbing the fall arrest energy.

The amount of energy absorbed by the deployment is the product of thedeployment load which must be applied to the load eye 55 to causedeployment to occur and the length of the coiled material 50 whichpasses around the rollers 51 and 52, allowing the load eye 55 to moveaway from the attachment element 53.

Normally, the materials and dimensions selected for the absorber will besuch that the amount of energy which will be absorbed by the energyabsorber by the time the end of the deployed material 50 is reached isgreater than the maximum expected fall energy in a fall arrestsituation. However, it is prudent in case of unforeseen circumstances,as well as being a requirement in many countries under the relevantlegislation as explained above, to provide an end stop to provide a hardstop to deployment of the deployable material 50 and movement of theload eye 55 when the end of the deployable material 50 is reached.Conveniently, this end stop can be provided by forming a portion SOBhaving an enlarged cross section at the end of the deployable material50, the portion 50B having a cross section which is too large to passthrough the hole 56 b in the guide bracket 56.

The cylindrical body section 60 is preferably formed as a cylindricalsteel cylinder which will not deform when the first predeterminedrotation load is applied. As the body section 60 rotates or pivots aboutits lower rim, the distance between the retaining element 53 and theload eye 55 will increase. Accordingly, in order for this rotation tooccur, some deployment of the deployable material 50 must occur.Further, bending and plastic deformation of the deployed material 50occurs as the energy absorber rotates. Because the deployable material50 deploys and bends, and so undergoes plastic deformation as the energyabsorber rotates, the rotated or reoriented energy absorber will remainin line with the applied force after the force has been removed even ifthe applied force never reaches the second deployment level to begindeployment of the deployable material 50. As a result, the position ofthe energy absorber will provide a clear visual indication that anapplied force at or above the first rotation level has been applied tothe absorber so that the absorber can be replaced and the other parts ofthe fall arrest system serviced as necessary.

The deployment of the deployable material 50 during rotation of theenergy absorber occurs at a lower applied load than the direct in-linedeployment which occurs when the energy absorber is oriented with theapplied load because of the leverage produced by the geometry of theenergy absorber. The distance of the pulling eye 55 from the base ofbody section 60 is greater than the radius of the body section 60.Accordingly, movement of the pulling eye 55 through a set distance as itpivots about the lower rim of the body section 60 requires deployment ofa length less than said set distance of the deployable material 50, sothat the applied load required to deploy the material 50 is reduced. Or,to put it another way, the distance of the pulling eye 55 from the baseof the body section 60 where the rim of the body section 60 contacts thebase plate 57 is greater than the distance of the rollers 51 and 52 sothat due to leverage the load applied to the material 50 to deploy it isgreater than the actual load applied at the pulling eye 55.

The body section 60 of the casing could be formed of a frangiblematerial and break up, or be formed of a plastically deformable materialand plastically deform rather than be rigid and rotate about its lowerrim when the applied force reaches the rotation level. Such arrangementswould still be able to provide a clear visual indication of an appliedforce at or above the rotation level. However, the use of a rigid bodysection is preferred because this allows some energy to be absorbed by areduced rate deployment of the deployable material 50 during rotation.As a result the length through which a falling user will fallunrestrained, and the total fall energy which must be absorbed, isreduced. As a result, the total distance fallen in a fall arrestsituation is minimised.

When deployment of the deployable material 50 has occurred the resultingextension of the energy absorber and the projection of the deployedmaterial will provide a clear visual indication that the energy absorberhas been subject to a load above the second deployment level.

The end stop should be able to resist a load equal to at least doublethe deployment load being applied to the energy absorber.

It is preferred that the deployable material be a length of stainlesssteel rod having a circular cross section and in this case the end stopcan be conveniently provided by an increased diameter end section 50B orby threading a nut having a larger diameter than the hole 56 b onto theend of the deployable material 50 similarly to the end stop arrangementof the first embodiment, so that the end stop cannot pass through thehole 56 b in the guide bracket 56.

Similarly to the first embodiment, because the deployable material 50still passes around the rollers 51 and 52 when the end stop is engaged,the capstan effect of the rollers 51 and 52 and the deployable material50 means that the load actually acting on the end stop is less than theload applied to the load eye 55.

The base plate 57 is shown in more detail in FIGS. 14 a to 14 c.

The base plate 57 is shaped as a substantially cone like shape made upof eight substantially flat faces arranged symmetrically.

The base plate 57 has a raised central portion 57 a with a central hole57 b through which the deployable material 50 can pass to allow theenergy absorber to be attached to the base plate 57. The base plate 57has a flat outer rim 57 c having a plurality of screw attachment holes57 d allowing it to be attached to a roof or other surface.

The central raised section 57 a includes a circular ledge 57 e on whichthe edge of the cylindrical portion 60 of the casing can rest.

Outside the circular ledge 57 e four retaining elements 57 f arearranged evenly spaced around the circumference of the ledge 57 e andprojecting a short distance above the surface of the ledge 57 e. Theretaining elements 57 f prevent sideways movement of the cylindricalelement 60 of the casing across the base plate 57. This ensures that thecylindrical section 60 does not move laterally as it rotates about itslower rim. If the cylindrical element 60 was able to slide laterallyacross the base plate 57, this could make the force level at whichrotation would occur less predictable and stable.

The base plate 57 can be secured to standard roof sheeting such as thatshown in FIG. 8 b by placing the base plate 57 so that two of theopposed flat edges of the outer rim 57 c of the base plate 57 lie alongtwo parallel ribs of the roof sheet. The base plate 57 can then be fixedto the roof sheet by screwing through the appropriate ones of holes 57 dthrough the roof sheeting along the top of the rib. The use of an oblongbase plate 57 is preferred because this allows a single base plate 57 tobe used on two different roof profiles having different ribs spacingswith one dimension of the oblong base plate 57 matching each of the roofprofiles. However, a square base plate could be used if preferred.

The faceted conical shape with a raised central section 57 a of the baseplate 57 provides greater resistance to deformation of the base plate 57by fall arrest forces acting on the base plate 57 through the energyabsorber. Further, the upward projection of the central section 57 c ofthe base plate 57 provides sufficient clearance for the retainingelement 53 and the end 50 a of the deployed material 50 bearing theretaining element 58. However, in order to minimise the couple generatedby forces applied through the energy absorber to the base plate 57,which couple will tend to apply an undesirable pull out force ratherthan a shear force to the screws used when the base plate 57 is attachedto a roof, it will normally be desirable to keep the upwardly projectingheight of the base plate 57 as low as possible.

The retaining element 53 distributes the forces acting through thedeployable material 50 across a relatively large area of the base plate57, reducing the possibility of local deformation or tearing of the baseplate 57.

As explained regarding the base plate of FIGS. 8 and 9, deformation orbuckling of the base plate 57 is undesirable, so the base plate shouldbe rigid enough to prevent buckling.

The use of two rollers causing plastic deformation of the deployablematerial 50 in opposite directions increases the amount of energy whichcan be absorbed by the energy absorber for a length of deployablematerial having a particular cross section. This allows the energyabsorber to be made more compact for a given energy absorbingrequirement.

The use of a rod having a circular cross section as the deployablematerial is preferred because this ensures that the forces required todeploy the material and to rotate the energy absorber are substantiallyindependent of the direction in which the load is applied to the eye 55.

Similarly to the casing 15 of the first embodiment the hemispherical cap59 acts as a protective shell between the energy absorber and roofduring absorber deployment, which avoids or minimises damage to theroofing and reduces the possibility of the absorber becoming coiled orfouled by awkward surfaces in the path of deployment.

The use of the spacer ring 61 is preferred, but is not essential.

The described embodiments relate to different designs of energyabsorber. However, all of the energy absorbers of the describedembodiments have the same fundamental operating principles so thatfeatures described with reference to one embodiment can generally beapplied to the other embodiments. In particular, the energy absorber ofthe third embodiment could be attached to a support structure by thebrackets described with reference to the first and second embodimentsand vice versa.

It is desirable in fall arrest systems to be able to easily identifyparts of the system which have been subject to heavy loads whether by afall arrest event or otherwise so that parts can be checked and replacedas necessary. This is particularly important in systems using plasticdeformation to absorb energy because the plastically deformed energyabsorbing parts must be replaced after each usage. All of theembodiments of energy absorber according to the invention provide clearand distinctive visual evidence that they have been subjected to anexcessive load by folding over when subject to the deformation load andthen deploying the deployable material so that the deployment structuremoves away from the base when subject to the deployment load.

As explained above, the described embodiments of energy absorber areable to operate reliably and to absorb a predetermined amount of energyat a predetermined deployment force a very wide range of applied forceangles. When employed as energy absorbers in anchor points for a fallarrest system fall arrest loads are normally applied at an angle closeto the roof plane but can potentially be in any direction in that plane.All of the embodiments described herein are able to operate in alldirections in a plane over a wide range of angles above and below theplane.

The energy absorbers themselves are also able to operate at all anglesabove the base or roof plane. However, the brackets described forattaching the energy absorbers to support structures may not beeffective at large angles above the base plane where the supportingstructure is a fragile structure such as a roof because of the pull outloads applied to the attachment screws. Accordingly, if the energyabsorbers are to be used in a fall arrest system attached to a fragilesupport structure which may be subject to loads at large angles abovethe base plane it may be necessary to use alternative attachmentmethods. However, such situations are very rare in fall arrest systems.

In the described embodiments the at least partially yielding materialcould have any cross section although stainless steel rod is preferredwith the rod having a circular section to enable the absorber to bepulled readily in any direction typically parallel to or above baseplate 11 or base 23. In practice it has been found that rod between 6 mmand 12 mm diameter is suitable. Whilst coiling of the helical energystore is shown as being coiled with a constant diameter, someembodiments may require the coil diameter to vary, particularly if theabsorber had an irregular shaped outer casing instead of casing 11 asshown in FIGS. 3 and 4. However, use of a helical store with a constantdiameter is preferred to ensure a near constant pull force on eye 9during plastic yielding. This is because a small diameter coil isexpected to provide greater resistance to plastic yielding around therollers shown in the figures than if the coiled diameter was greater,and so produce a greater deployment force.

Ideally, the yielding process of the at least partially yieldingmaterial should be free from friction effects in order to achieve asnear constant and predictable pull force on eye 9 throughout deployment.To assist with this, the at least partially yielding material and/orroller and/or axes of rotation of rollers may be coated in a lowfriction material such as molybdenum disulphide or plated in a coatingsuch as silver or tin so that such surface coating becomes sacrificed onyielding of material and has the effect of reducing friction.

The described embodiments differ in their details but they are linked bycommon operating principles. Accordingly, it will be understood by theperson skilled in the art that the technical features described withreference to one embodiment will normally be applicable to otherembodiments.

Where the invention has been specifically described above with referenceto these specific embodiments, it will be understood by the personskilled in the art that these are merely illustrative althoughvariations are possible within the scope of the claims which follow.

1. An energy absorber comprising: a support attachment member to attachthe energy absorber to a supporting structure in an initial orientation;a load attachment member; a coiled length of plastically deformablemetal wire having a first end and a stop formed on a second end; and adeployment member, attached to one of the support attachment member andsaid load attachment member, the other one of the support attachmentmember and load attachment member being connected to the first end ofthe metal wire, the deployment member initially cooperating with aregion of the metal wire adjacent to the first end thereof, thedeployment member further provided with at least one guide roller aboutwhich the metal wire is entrained; wherein when a force exceeding afirst predetermined limit is applied to the load attachment member, thesupport attachment member and load attachment member orient in alignmentwith the direction of the applied force, when the force exceeds a secondpredetermined level, the deployment member roller periphery translatesalong the length of the metal wire causing the metal wire to locallystraighten as the deployment member translates along the length of themetal wire absorbing energy.
 2. An energy absorber as claimed in claim 1in which the coil is a helical coil.
 3. An energy absorber as claimed inclaim 1, in which the end stop is able to resist a load of at leasttwice the second predetermined tensile load.
 4. An energy absorber asclaimed in claim 1, in which the end stop prevents the yielding materialbecoming released from the deployment member.
 5. A fall arrest systemincorporating an energy absorber according to claim
 1. 6. An energyabsorber comprising: a support attachment member to attach the energyabsorber to a supporting structure in an initial orientation; a loadattachment member; a coiled length of plastically deformable metal wirehaving a first end and a stop formed on a second end; and a deploymentmember, attached to one of the support attachment member and said loadattachment member, the other one of the support attachment member andload attachment member being connected to the first end of the metalwire, the deployment member initially cooperating with a region of themetal wire adjacent to the first end thereof, the deployment memberfurther provided with at least one guide roller about which the metalwire is entrained; wherein when a force exceeding a first predeterminedlimit is applied to the load attachment member, the support attachmentmember and load attachment member orient in alignment with the directionof the applied force, when the force exceeds a second predeterminedlevel, the deployment member roller periphery translates along thelength of the metal wire causing the metal wire to locally deform andabsorb eneray with the force required to translate the deployment memberalong the length of the metal wire being substantially constant untilthe stop abuts the deployment member.
 7. A fall arrest systemincorporating an energy absorber according to claim
 6. 8. An energyabsorber as claimed in claim 6 in which the coil is a helical coil. 9.An energy absorber as claimed in claim 6, in which the end stop is ableto resist a load of at least twice the second predetermined tensileload.
 10. An energy absorber as claimed in claim 6, in which the endstop prevents the yielding material becoming released from thedeployment member.